(1) Field of the Invention
The present invention relates to the field of assisting the piloting of rotary wing aircraft.
The present invention relates to a method and to a device for estimating the instantaneous mass of a rotary wing aircraft.
(2) Description of Related Art
A rotary wing aircraft conventionally comprises a power plant having at least one engine, generally a turboshaft engine, and a main power transmission gearbox, the power plant acting via the main gearbox to drive at least one main rotor for providing the aircraft with lift and possibly also with propulsion, and possibly also to drive an anti-torque rotor. By way of example, an anti-torque rotor may be provided by a tail rotor of the aircraft or indeed by two propulsive propellers situated on either side of a fuselage of the aircraft.
In operation of the aircraft, various characteristic parameters are generally monitored by means of several instruments situated on an instrument panel of the aircraft. These characteristic parameters are representative of the current operation of the aircraft and in particular of its power plant and/or of each turboshaft engine.
By way of example, these characteristic parameters may be the speed of rotation Ng of the gas generator of each engine, the gas ejection temperature T4 at the inlet to the free turbine of each engine, and the driving torque Cm from each engine.
For physical reasons, there exist limits on these characteristic parameters, which limits need to be taken into account at all times while the aircraft is in operation. These various limits may depend on external conditions and also on the mode of operation of the aircraft.
While the aircraft is in operation, its pilot needs to monitor the current values of these characteristic parameters continuously and to compare them with their respective limits.
These limits generally differ depending on the stage of flight of the aircraft and/or on external conditions, such as altitude and temperature, for example. Specifically, the maximum power that can be delivered by the power plant differs and the length of time for which it is available may also be limited depending on each stage of flight and/or depending on external conditions and indeed on the mode of operation of the power plant.
Furthermore, the instantaneous mass of the aircraft is a parameter that is important for determining certain limits and/or certain current values of these characteristic parameters, such as the power that each engine of the power plant needs to deliver in order to perform the flight or indeed to perform some particular maneuver. In particular, the power plant needs to be capable of supplying sufficient mechanical power to ensure that the lift from the main rotor supports at least the instantaneous mass of the aircraft, thus providing the aircraft with lift.
At present there exist several methods of estimating the mass of the aircraft.
The mass of the aircraft is generally estimated before the aircraft takes off and flies, e.g. by summing the empty mass of the aircraft plus the on-board mass of fuel, plus the mass of the crew, and the mass of the transported payload. This estimate of the mass of the aircraft prior to takeoff is then used during the flight of the aircraft. This estimate of the mass of the aircraft then takes no account of fuel being consumed by the aircraft. This estimate of the mass of the aircraft is thus constant and remains unchanged throughout the flight of the aircraft, so its difference relative to the real instantaneous mass of the aircraft increases as a flight continues. Consequently, this estimate of the mass of the aircraft is an overestimate, and it therefore leads to the performance of the aircraft not being optimized.
It is also possible for the mass of the aircraft to be estimated while the aircraft is in flight, generally by subtracting the quantity of fuel that has been consumed from the estimate of the mass of the aircraft prior to takeoff.
Nevertheless, this estimate of the mass of the aircraft is generally not very accurate, and it leads to information about the performance of the aircraft that is not optimized. It is general practice to use a safety margin when determining this estimate of the mass of the aircraft, and as a result the estimated mass of the aircraft is an overestimate. By way of example, this safety margin may take account of approximations in determining the mass of the aircraft.
This overestimate of the mass errs on the safe side, so the mechanical power that the aircraft genuinely needs for flight is less than the necessary power as determined on the basis of this estimate of the mass of the aircraft. However, for certain maneuvers that require a large amount of mechanical power, this overestimated power requirement may be greater than the maximum power available from the power plant, whereas the mechanical power that is genuinely required is less than the maximum power available for each engine. Consequently, such maneuvers are not performed by the pilot who believes, wrongly, that the power plant cannot deliver sufficient total power. This estimate of the mass can thus lead to the flight envelope of the aircraft being reduced to a greater or lesser extent.
Nevertheless, it is also possible for this estimate of the mass of the aircraft to be an underestimate, particularly as a result of wrongly identifying the on-board mass not including fuel, e.g. as constituted by the passengers of the aircraft and their baggage. Such an underestimate of the mass of the aircraft leads to a safety risk during flights of the aircraft, unlike an overestimate, and accidents have occurred as a result of errors of this type.
The mass of an aircraft can also be determined by making use of measurements of the environment and/or of components of the aircraft.
By way of example, Document EP 2 461 142 describes a device for determining the takeoff mass of an aircraft, in particular by using atmospheric parameters relating to the environment of the aircraft, flight parameters of the aircraft and relating to its engines, and also performance curves for its engines. That method is adapted to the aircraft performing stabilized and horizontal flight, i.e. level forward flight at constant speed, or hovering flight.
Likewise, Document EP 0 502 811 describes a device for determining the mass of an aircraft that makes use of the distance between its fuselage and the blades of its main rotor together with the speed of rotation of the blades.
Also known is Document U.S. Pat. No. 5,987,397, which describes a device for estimating the mass and the center of gravity of an aircraft as a function of measurements of flight parameters and by using a neural network system. The neural network applies a relationship between the mass of the aircraft and its flight parameters using an algorithm that is non-linear, which relationship is established beforehand on the basis of test flights.
Furthermore, Document GB 2 137 153 describes a device for determining the mass of an aircraft when its landing gear is in contact with the ground and as a function of the upward thrust developed by the main rotor of the aircraft.